Existence Form And Risk Assessment Of Heavy Metals In The Deposits Formed In Drinking Water Distribution Systems

The accumulation of inorganic contaminants in drinking water distribution systems(DWDS)can greatly threaten water quality and safety.Based on the structural characterization of pipe network sediments,this work mainly focused on the accumulation level,speciation and risk assessment of inorganic contaminants found in pipe scales and loose deposits in DWDS.Consensus-based sediment quality guidelines(CBSQGs)based on the concentration and global contamination factor(GCF)and risk assessment code(RAC)based on the existing form and were adopted for the potential health risk assessment of inorganic contaminants.The work also discussed the effects of water quality parameters(pH,humic acid and polysilicic acid)on the release of heavy metals in pipe scale,and analyzed the re-adsorption of heavy metals in pipe scale.The work also studied the adsorption kinetics of heavy metals by pipe scale.This topic finally put forward feasible suggestions for optimized maintenance of pipe network and safety of drinking water.The main research results were as follows:(1)The correlation analysis of inorganic contaminants in pipe scales and loose deposits showed that there was significant correlation between Al and Mn,which indicated there was a cooccurrence behavior of Al and Mn.In addition to the possible interactions between Al and Mn,Ba,Cu and As were possibly accumulated during the formation of Al and Mn commixtures.(2)The Tessier sequential extraction method was used to study the speciation of inorganic contaminants in fourteen samples(six pipe scale samples,eight loose deposit samples)collected from real DWDS.The results of the five-stage procedure indicated that Mn,Cu,Pb,Zn,Ni,Co and Ba in the samples were mainly associated with the FeMn oxides fraction.And it indicated Fe-Mn oxides might play an important role in the accumulation and release of these inorganic contaminants.Fe,Al,As,Cr,V and Cd were mainly present in the residual fraction,which indicated these metals had low mobility.There was no significant difference in the speciation of inorganic contaminants in pipe scales and loose deposits.(3)According to the CBSQGs evaluation,heavy metals in loose deposits were more harmful,and Ba exhibited the highest risk among all heavy metals.The GCF results demonstrated that most of the sample sites had a certain environmental risk.The RAC results showed that high risk mainly resulted from Cd both in pipe scales and loose deposits.Three risk assessment methods showed that heavy metals in loose sediments were more harmful than in pipe scales.(4)The study on the release of heavy metals in pipe scale showed that pH had little effect on the dissolution and release of heavy metals in pipe scale.Both humic acid and polysilicic acid had certain effects on the release of heavy metals in pipe scale.The readsorption of heavy metals by pipe scale showed that adsorption isotherms of the heavy metals by pipe scale were different.